Pleiotropic Effects Of Hla-dq Mutation In Predisposition To T1 Diabetes Mellitus And Celiac Disease

Human leukocyte antigen (HLA) is a region on chromosome 6. There are 224 identified gene loci (of which 128 are protein coding) meaning that HLA has the highest gene density of any genomic region. 40% of expressed genes are implicated in immune system function and the vast majority of autoimmune diseases are associated with HLA gene loci (Beck, et al. 1999). Mutations in HLA-DQ cause a CD4 cell (also known as T-helper cell) response against cells presenting 'self' antigens, leading to production of autoantibodies that causes the destruction of the affected cell type (Gough & Simmonds, 2007). Mutations in the HLA-DQ gene have many pleiotropic effects and it is implicated in the onset of numerous illnesses. Examples of such T-cell mediated autoimmune diseases include: type 1 diabetes mellitus, celiac disease, Grave's disease, multiple sclerosis, rheumatoid arthritis, any many more (Pocock, et al. 2013).

HLA-DQ is a centromeric gene in the HLA region, producing class 2 HLA molecules. In an immune system reaction, antigens are broken down to amino acid components by antigen presenting cells (APCs) and then bound by these HLA class 2 molecules. This forms a class 2-peptide complex that is presented on the cell surface and recognised by CD4 cells; triggering an immune response by activating antibody production in B cells, or via activation of T-killer cells (Gough & Simmonds, 2007). Autoimmune diseases occur when antibodies are raised against 'self' protein markers; these are HLA molecules that have not bound to antigenic amino acids, but are unaltered or presenting endogenous host cell peptides. Autoantibody production is normally prevented as any CD4 cells that react with such HLA molecules are usually destroyed and not released into the circulation (Pocock, et al. 2013). The mutant HLA-DQ gene show pleiotropy as it is strongly implicated in both conditions of interest and is considered a major predisposing genetic factor, however it is important to understand that it does not directly cause them. A cohort of genetic variations change the body's natural immune response and interact with environmental stimuli to cause the onset of such diseases.

The protein binding site of HLA-DQ contains 2 deep pockets - a type of structural motif within the 3D structure. Mutations in the P1 and P9 pocket sites interact inappropriately with endogenous proteins to illicit an enhanced immune response against the host tissue. The HLA-peptide complexes produced in these mutations are more stable and present at the cell surface more intensely. Therefore stimulating a stronger CD4 cell response and the production of autoantibodies, or activation of cytotoxic T-cells. This was demonstrated in experiments using synthetic peptides derived from the Epstein-Barr virus, modified to present with differing intensities, and with a high affinity for HLA molecules. Immunofluorescence was used to quantify the intensity of cell surface peptides; cytotoxicity was measured by the percentage of specific lysis of viral cells in culture. The results showed that small modifications in HLA residues causing conformational changes to the HLA-peptide complex structure affected recognition by CD4 cells. A more stable complex was recognised more readily and therefore elicited a stronger immune response (Micheletti, et al. 1999).

Two possible HLA-DQ mutations are most common in type 1 diabetics and coeliacs - DQ8 and DQ2. These variants are positively charged at the P9 pocket site as they have an alanine at residue beta57, rather than the normal negatively charged aspartic acid (Wucherpfennig, 2001). Consequently the HLA-DQ bonding interactions with peptides are altered, which in turn effects the immune response. The onset of type 1 diabetes mellitus and celiac disease follows different immune reaction sequences but this gene is intrinsically involved in both, demonstrating the pleiotropic nature of HLA-DQ.

In type 1 diabetes mellitus, the altered HLA molecule changes the bonding pattern within the DQ-insulin peptide complex resulting in an increased half life, and presentation of host peptides at the cell surface of APCs (Wucherpfennig, 2001). This stimulates a strong CD4 cell response and production of antibodies by B-cells against pancreatic beta cells. Tissue phagocytes are then produced that destroy these insulin secreting beta cells. Autoimmunity such as this is referred to as type 2 - antibody dependent cytotoxic hypersensitivity (Pocock, et al. 2013).

Celiac sufferers produce transglutaminase antibodies in response to ingestion of gluten. These antibodies alter the structure of the enzyme tissue transglutaminase, resulting in the conversion of glutamine residues 208 and 216 to glutamic acid. The altered enzyme binds more strongly to HLA-DQ molecules, at sites within the P1 and P9 pockets (Wucherpfennig, 2001). Future ingested gluten is recognised as a foreign antigen by CD4 cells due to the APCs presenting transglutaminase peptides from the original sensitisation. Immune system memory cells are activated; proliferating and releasing inflammatory cytokines that attack the inner gut lining. This form of autoimmune disease in termed type 4 - cell mediated delayed hypersensitivity (Pocock, 2013).

There are suggestions that gluten consumption and associated mutations in celiac disease are an additional factor in type 1 diabetes mellitus onset. Changes in immune system function within the gut could go on to affect the pancreatic immune system. HLA-DQ is a candidate gene for such mutations (Smyth, et al. 2008). This proposed pleiotropic effect is supported by the fact that, c. 8.0% of type 1 diabetics are also celiacs, compared with c. 0.25% of the general population (Wucherpfennig, 2001).

The pleiotropic effect of HLA-DQ alleles is evident. They are implicated in the onset of both type 1 diabetes mellitus and celiac disease, demonstrating that variations in this gene can have several phenotypic effects. HLA-DQ mutations create a shared predisposing genetic basis by affecting autoimmunity and inflammation. This alters the response to future specific epigenetic changes and environmental factors leading to development of either type 1 diabetes mellitus or celiac disease (Smyth, et al. 2008).

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References:

Beck, S., Geraghty, D., Inoko, H., Rowen, L. (1999). Complete sequence and gene map of a human major histocompatibility complex. Nature. 401, p921-923.

Gough, S.C.L. & Simmonds, M.J. (2007). The HLA region and autoimmune disease: associations and mechanisms of action. Current Genomics. 8, p453-465.

Micheletti, F., Bazzaro, M., Canella, A., Marastoni, M., Traniello, S., Gavioli, R. (1999). The lifespan of major histocompatibility complex class 1/peptide complexes determines the efficiency of cytotoxic T-lymphocyte responses. Immunology. 96, p411-415.

Pollock, G., Richards, C., Richards, D. (2013). Defence against infection: the immune system. In: Richards, D,. J, Woodward. Human physiology. 4th ed. Oxford: Oxford university press. p341-358.

Smyth, D., Plagnol, V., Walker, N., Cooper, J., Downes, K., Yang, J., Howson, J., Stevens, H., McMannus, R., Wijmenga, C., Heap, G., Dubois, P., Clayton, D., Hunt, K., van Heel, D., Todd, J. (2008). Shared and distinct genetics variants in type one diabetes and celiac disease. The New England journal of medicine. 359, p2767-2777.

Wucherpfennig, K. (2001). Insights into autoimmunity gained from structural analysis of MHC-peptide complexes. Current opinion in immunology. 13, p650-656.